The major objective of this project is to clarify the mechanism by which all-trans vitamin A is isomerized to the 11-cis form during dark adaptation. Once that the mechanism is known, the relationship of the isomerization reaction to various pathological conditions, such as retinitis pigmentosa in humans and retinal dystrophy in rats, can be probed. Our studies thus far have provided the following leads: 1) by use of high pressure liquid chromatography, physiologically important isomers of retinaldehyde, retinal oxime, retinol and retinyl ester have been separated and quantitated, 2) the net conversion of all-trans to 11-cis vitamin A has been demonstrated in the bovine eye up in vitro, and 3) the site of isomerization seems to be the rod cell, and probably is the rod outer segment. Specific aims are: 1) to study the cellular site and nature of the all-trans: 11-cis isomerization of vitamin A in the eye, 2) to investigate a possible linkage between the isomerization reaction and energy donating systems, 3) to examine in detail the mechanism of the isomerization reaction, and 4) to probe the possible relationships between defects in isomerization and degenerative diseases of the retina. Our working hypothesis is that all-trans retinaldehyde, when bound to a specific binding protein in the rod cell, is isomerized to the 11-cis form in the presence of a suitable energy donating system, and then is transferred to opsin.